Fast stopping method for induction motors operating from variable frequency drives

ABSTRACT

A control for an induction motor operated by a variable frequency drive (VFD) includes a control unit adapted to control the VFD in a normal mode or a stopping mode. The normal mode comprises controlling the VFD at a select commanded operating frequency to control speed of the induction motor. The stopping mode comprises controlling the VFD at a substantially lower commanded operating frequency to operate the induction motor at a high negative slip condition to rapidly stop rotation of the induction motor.

FIELD OF THE INVENTION

This invention relates to variable frequency drives and, moreparticularly, to a fast stopping method for an induction motor operatingfrom a variable frequency drive.

BACKGROUND OF THE INVENTION

A motor drive system, in one known form, comprises an AC sourcesupplying three-phase AC power to a variable frequency drive (VFD). TheVFD includes an AC/DC converter connected by a DC link or bus to a DC/ACconverter. The DC/AC converter may comprise a pulse width modulatedinverter using insulated GATE bipolar transistors (IGBTs).

Speed and torque control of induction motors using VFDs has becomeuniversally accepted in the industry. Speed control includes stopping aswell as reducing speed of a rotating load. Many applications requirerapid stopping of a rotating motor. There are two (2) common stoppingmethods used in adjustable speed drives (ASDs). These are coasting anddecelerated stop. In the coasting method, the control signals forturning on and off the inverter IGBTs are turned off and no voltage isprovided to the motor. The motor coasts to a stop. The time taken forthe motor to come to rest depends upon the inertia of the rotor-loadcombination. If the inertia is not large, then the rotor takes a longtime to come to rest. In the decelerated stopping method, the motor iscommanded to operate at a reduced frequency, and thus speed. Thecommanded operating frequency (speed) is gradually reduced to bring themotor to rest. Due to the inertia of the rotor-load combination, thespeed cannot be reduced instantaneously. Generally, the user can selecta predetermined rate of deceleration, which can be adjusted depending onthe application. Since the commanded speed is lower than the actualrotating speed of the rotor-load, the motor starts behaving like aninduction generator. In other words, the motor enters negative slipoperation. Slip is defined as the ratio of the difference between thecommanded speed and actual speed to the commanded speed. The commandedspeed is also known as synchronous speed ω_(s). Mathematically, slip isdefined as follows: $\begin{matrix}{s = \frac{\omega_{s} - \omega_{r}}{\omega_{s}}} & (1)\end{matrix}$

In equation (1), ω_(r) is the rotor speed or the actual rotating speedof the rotor-load combination. Note that slip, s, is a dimensionlessquantity.

The mechanical energy in the rotor-load inertia is converted intoelectrical energy by the induction generator action. This energy istypically absorbed by the DC bus capacitors present in all ASDs. Thevoltage across the DC bus rises. Eventually, the excess energy isdissipated in the bleeding resistors present across the DC buscapacitors. In many cases, the energy in the moment of inertia is verylarge and it takes a long time for the energy to bleed off into bleedresistors. This delays the time it takes to stop a rotating load withlarge inertia. If the user selects a low deceleration time to bring therotor-load combination to a rapid stop, then the rate of increase involtage across the DC bus capacitors could be higher than the timeconstant of the bleed resistor and DC bus capacitor combination. Thiscould result in an over-voltage trip condition at which time the VFDreverts to a coasting method to stop. Nuisance trips similar to thosedescribed above are highly undesirable and result in loss of productiondue to machine downtime. In order to circumvent this situation, manyusers employ external IGBT-resistor combinations. Such a unit is knownas a “brake unit”. The IGBT is turned on when the voltage across the DCbus exceeds a predetermined value. The excess energy is thus dissipatedin the external resistors. The size and cost of the externalIGBT-resistor combination is an added burden to the end user. The methodof employing external resistors to achieve relatively faster stoppingtime is known as dynamic braking and the resistors are known as DBresistors.

The present invention is directed to solving one or more of the problemsdiscussed above in a novel and simple manner.

SUMMARY OF THE INVENTION

In accordance with the invention, there is provided a fast stoppingmethod for induction motors operating from variable frequency drives.

It is an object of the invention that the braking method does not causean over voltage trip in the VFD.

It is another object of the invention that the braking method does notrequire the use of additional brake-units or regenerative units.

It is still another object of the invention that the method does notrequire use of a tachometer or encoder feedback.

It is still a further object of the invention that the method has astopping time that is better or comparable to the method usingadditional brake units or regenerative units.

In accordance with the invention, the above objects are achieved in afast stopping method known as large slip braking (LSB). The proposedmethod relies upon maintaining large slip between the commanded speedand the actual rotor speed to achieve fast stopping times.

Further features and advantages of the invention will be readilyapparent from the specification and from the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a generalized block diagram of a motor drive system includinga control implementing a fast stopping method in accordance with theinvention;

FIG. 2 is a schematic diagram of a variable frequency drive of thesystem of FIG. 1;

FIG. 3 is an equivalent circuit of an induction motor operating at aslip s;

FIG. 4 is a curve illustrating torque-slip and power in an equivalentvariable resistor of a typical induction motor;

FIG. 5 is a flow diagram of the fast stopping method implemented in thecontrol of FIG. 1; and

FIG. 6 is a time diagram representation of the proposed fast stoppingmethod.

DETAILED DESCRIPTION OF THE INVENTION

Referring initially to FIG. 1, a motor drive system 10 is illustrated.The motor drive system includes an AC source 12, a variable frequencydrive (VFD) 14 and a control 16 for driving an induction motor 18. TheAC source may comprise a drive or the like developing three-phase ACpower on feeder conductors labeled R, S and T. The AC source 12 isgrounded. The VFD 14, as described more particularly below, converts theAC power from the feeder conductors R, S and T, to DC power and convertsit back to AC power at a select frequency which is then impressed acrossterminals U, V and W. The terminals U, V and W are connected to three(3) feeder conductors 20, 21 and 22 to drive the motor 18.

Referring to FIG. 2, a schematic diagram illustrates a typical circuitimplementation for the VFD 14. The VFD 14 includes an AC/DC converter 24connected by a DC bus 26 to a DC/AC converter 28. Particularly,according to the illustrated embodiment of the invention, the AC/DCconverter 24 comprises a flow wave bridge rectifier circuit ofconventional construction which is operable to convert three-phase ACpower to DC power. The DC bus 26 includes a conventional filter 30. TheDC bus 26 has rails labeled “+” and “−”. The DC/AC converter 28comprises an inverter section. Particularly, the inverter section 28comprises a pulse width modulation (PWM) inverter, using insulated GATEbipolar transistors (IGBTs) 29. The six (6) IGBTs 29 are connected in athree-phase bridge configuration to the DC Bus to develop power at theterminals U, V and W. The IGBTs 29 are pulsed width modulated by signalson lines 32 from the control 16, see FIG. 1, using a conventionalcontrol scheme. Particularly, the PWM inverter 28 is controlled tocreate a sinusoidal effect for the induction motor 18. The pulsefrequency used is fixed. The pulse width is varied to varied sinusoidalfrequency.

Referring back to FIG. 1, the control 16 senses various parameters usedin controlling the IGBTs 29 via signals on the line 32. Some of thesensed parameters are conventional, but not discussed herein as they donot relate specifically to the fast stopping method disclosed herein. Avoltage sensor 34 senses DC bus voltage and is connected via a line 36to the control 16. Current sensors 38 sense current on each of theconductors 20, 21 and 22 and are likewise connected to the control 16via lines 40.

As is apparent, the control 16 is operable to pulse width modulate theIGBTs to vary sinusoidal frequency to control speed of the motor. Thetechniques for doing so are well-known and are not discussed herein. Thepresent invention relates particularly to a fast stopping method for aninduction motor 18 using a VFD 14.

In accordance with the invention, a large slip braking method is usedfor providing fast stopping of the motor 18. In order to achieve thefast stopping, it is important to understand that all methods have torely upon converting mechanical energy stored in the form of inertia toelectrical energy. The fast stopping method dissipates most of theelectrical energy in the rotor circuit. This is explained via the motorequivalent circuit of FIG. 3 and the torque-slip curves shown in FIG. 4.

Referring initially to FIG. 3, the voltage V₁ is the per-phase inputline to neutral voltage. The resistor R₁ represents the statorresistance. I₁ represents stator current. X₁ represents statorinductance. The block Z_(M) represents the magnetizing inductance of themachine and core lost components. The resistance R₂ represents reflectedrotor resistance at the stator side. The current I₂ represents rotorcurrent. Inductance X₂ represents reflected rotor inductance. Finally,the variable resistance term R₂ (1−S)/S represents the reflected powercomponent of the motor.

The rotor resistance R₂ is independent of operating slip s. X₂ is givenby:

i X₂=2*π*f ₃ *L ₂  (2)

f_(s) is the synchronous frequency. L₂ is the inductance of the rotorcircuit and includes the rotor leakage inductance. The power dissipatedin the variable resistor term equals the electromagnetic output power.If slip is negative, then this power is negative also. This means thatthe machine enters generator mode of operation.

The proposed fast stopping method involves operating the induction motor18 in the high negative slip region. Under these conditions, from theequivalent circuit shown in FIG. 3, the electromagnetic power output I₂²R₂(1−S)/S is negative since slip is negative. The available kineticenergy of the rotor-load combination provides for the mechanical lossesand the remaining is converted into electrical energy by the inductiongenerator action, which is represented by the variable resistance partof the motor equivalent circuit in FIG. 3.

The available electric power is a function of operating slip as shown inthe power diagram in FIG. 4. A large part of this electrical power isdissipated in the rotor resistance R₂, a smaller part of it isdissipated in the stator resistance, R₁, and as core losses in the rotorand stator. The remaining is returned to the electrical source, i.e.,the inverter 28 in the illustrated embodiment of the invention. Inaccordance with the invention, the operating slip is chosen so that therotor and hence the stator current amplitudes are such that theavailable electric power is consumed by the rotor resistance, the statorresistance, and the motor core and nothing is returned back to thesource. In other words, the induction generator is operated at a verylow efficiency point. The operating slip at which point this happens canbe approximately derived as follows: $\begin{matrix}{{{I_{2}^{2}R_{2}*\left( \frac{1 - s}{s} \right)} + {I_{2}^{2}R_{2}} + {I_{1}^{2}R_{1}} + P_{core}} = 0} & (3)\end{matrix}$

Using an approximate equivalent circuit of the motor 18, the statorcurrent I₁ can be assumed to be equal to the rotor current I₂. It isimportant to note from equation 3 that the slip where the net powerreturn to the source is zero depends strongly on the machine parameters.Further, if the core loss is assumed to be half of the total copper lossin the machine, and the stator resistance is approximately equal to halfthe rotor resistance, then on a rough approximation, the slip at whichzero power is returned to the source occurs at $\begin{matrix}{{{{I_{2}^{2}R_{2}*\left( \frac{1 - s}{s} \right)} + \left( {I_{2}^{2}\left( {15R_{2}} \right)} \right) + \left( {0.75I_{2}^{2}R_{2}} \right)} = 0}{{{\frac{1 - s}{s} + 2.25} = 0};{s = {- 0.8}}}} & (4)\end{matrix}$

As described, equation 4 is only an approximation but indicates thatsuch a point exists and occurs when the machine operates in thegenerator mode with a high value of negative slip. For the examplegiven, if the induction motor is deliberately operated at a speed whichis about fifty-five percent the speed of the rotating mass, then most ofthe mechanical energy in the rotating mass is absorbed by friction,rotor and stator winding losses, and core loss. Since this process takesplace with high stator current, the mechanical energy is quicklyconverted and dissipated. In the absence of a constant mechanical primemoving action, the rotor-load mass will come to a fast stop. Thus, thepresent invention provides quick stopping by forcing the motor tooperate as an induction generator with relatively high values of slipuntil the rotor-load combination has come down to a very low speed.

Under very high slip conditions, the amplitude of current increasestremendously and reaches the current locus limit given by E₂/X₂. Thephase relationship between rotor current and induced voltage reachesalmost ninety electrical degrees denoting low values of regenerativepower being produced by the motor 18.

As the motor slip reduces, the commanded speed comes close to the actualspeed. At zero speed they are equal. Hence, there is always a point inthe speed curve where the slip is very low. In accordance with theinvention, the low-slip point is forced to occur at a low speed at whichpoint there is low mechanical energy remaining in the system.

The fast stopping method in accordance with the invention employs thesimple V/F control method.

The control 16 of FIG. 1 comprises a microcontroller or the like andassociated memory and other devices operating under accordance with acontrol program for controlling the VFD 14. Particularly, the control 16analyzes the various input parameters and controls switching of theIGBTs 29 to varied sinusoidal frequency and thus motor speed.

Referring to FIG. 5, a flow diagram illustrates a program implemented inthe control 16 of FIG. 1 for implementing the fast stopping methodaccording to the invention. At startup, various setup parameters areinitialized in block 50. These parameters include jump frequencies f₂and f₃. The jump frequencies are frequencies below rated frequency tocause the motor to operate under high negative slip. These values may beuser-selectable. In one embodiment, the jump frequency f₂ is set to beone-third of the base operating frequency. The jump frequency f₃ is setto be one-third of the jump frequency f₂. Other parameters that arepreset are a base block time t₁ and fallback times t₂ and t₃. The baseblock time is used to de-energize the IGBT 6 for a select amount of timeprior to change in commanded frequency. This is used to reset thecontrol 16. The base block timer time t₁ may be, for example, in theorder of one hundred milliseconds. Additional initialized parametersinclude the V/F pattern during the stopping method and DC Busover-voltage trip level OV2 and per-phase under current trip level UC1.

A decision block 52 determines if the motor 18 is operating in a reversedirection. If so, then a reverse flag REV is set in block 54. If not,then the REV flag is reset in block 56. From either block 54 or 56, adecision block 58 determines if the stopping method has been initiated.If not, then control returns back to the main operating program (notshown) in block 60. If the stopping method has been initiated, thencontrol proceeds to implement the fast stopping method in accordancewith the invention.

The fast stopping method is initiated beginning in block 62 whichdetermines if the current output frequency from the inverter 28 isgreater than the jump frequency f₂. If so, then the base block timer isinitiated for the time t₁ in block 64. As discussed above, this turnsoff all of the IGBTs 29. The inverter 28 is re-engaged to run at thejump frequency f₂ in block 66. This causes the motor 18 to operate witha high value of negative slip as the jump frequency f₂ is on the orderof one-third of the base operating frequency. A decision block 68evaluates for various conditions to determine when control at the firstjump frequency f₂ should terminate. These conditions include whether theDC bus voltage is greater than OV2, or if the output current is lessthan UC1, or the fallback timer t₂ has elapsed. If none of theseconditions have been satisfied, then the output frequency is maintainedat f₂ in block 70 and the control loop loops back to the decision block68.

Once any of the conditions in the block 68 has been satisfied, then thecontrol proceeds to a block 72 which initiates the base block timer fort₁ seconds. Again, this has the effect of turning all of the IGBTs off.The inverter 28 is then re-engaged to run at the second jump frequencyf₃ in block 74. The fallback timer t₂ is also reset. A decision block 76then waits for various conditions to occur. These conditions are similarto those in the block 68, except that the second fallback timer t₃ isused, instead of the timer t₂. If none of these conditions occur, thenthe output frequency is maintained at the second jump frequency f₃ inblock 78 and the control loops back to the decision block 76. Once anyof the conditions have been satisfied, then control advances to a block80 which continues deceleration using a second deceleration ramp inblock 80. The second fallback timer t₃ is also reset. The control 16includes various ramp selections. A first deceleration ramp is regulardeceleration stopping as known. The second deceleration ramp is usedwith the fast stopping method to decelerate from the frequency f₃ tozero hertz. Once the frequency has ramped down to zero, then the processstops in block 82.

As is apparent, the operating frequency may be lower than the first jumpfrequency f₂ when the fast stopping method is initiated, as determinedat the decision block 62. If so, then a decision block 84 determines ifthe output frequency is greater than f₃. If so, then control advances toa block 86 that initiates the base block timer for t₁ second. Theinverter 28 is then reengaged to run at f₃ at the block 74, discussedabove. If the output frequency is not greater than f₃, as determined atthe decision block 84, then the output frequency is less than or equalto f₃, as indicated at the block 88. The control loop then advancesdirectly to the block 80 to implement the second decel ramp, discussedabove.

The fast stopping method is now discussed under the assumption that amotor with a large inertial load is operating at 120 Hz. The motor-loadis rotating at approximately 3600 rpm (4-pole motor running at 120 Hz)and must be brought to a dead stop. At the desired time of stopping, thecontrol signals to the output IGBTs 29 are removed to initiate the baseblock. The motor starts to coast. After the programmable coasting timet₁, the motor is reenergized with the command speed set at the desiredlow speed level f₂ which is typically one-third of the operating speedat the time of initiating a stop command. The timer t₂ starts countingdown simultaneously. The DC bus voltage and output current for thesecond jump are monitored, as shown in FIG. 6. When the motor slows downand the slip reduces in magnitude, the output voltage will tend toincrease and if it crosses the preset level then a second base block isinitiated, again for t₁ seconds. After the programmable coasting time,the motor is reenergized with the second jump speed set at the desiredlow speed level, f₃, which is typically one-third of f₂. The timer t₂ isreset and the timer t₃ is started.

In some instances, the DC bus voltage does not rise fast enough or isnot detectable for initiating the second jump. In such a circumstance,the motor speed comes in line with the commanded speed and startsrunning at f₂ instead of coming to a stop. In order to expedite thestopping time under such circumstances, the output current is monitoredas discussed above. If the output current falls below ten percent ofnominal value, and the second jump has not yet been initiated then theprocess is started for the second jump by initiating the second baseblock as shown in FIG. 5 at the block 68. In other words, the secondjump will take place if either the DC bus voltage rises above tenpercent of the nominal voltage or if the output current falls below tenpercent of the nominal output current.

In rare instances, neither the DC bus voltage rise is detectable nordoes the current fall below the preset low level. In such a condition,the fallback timer t₂ that started counting down after the firstfrequency jump times itself out. If the output frequency is still higherthan zero and t₂ has timed itself out, then the second jump process isstarted by initiating the base block at the block 72 of FIG. 5, asdiscussed above.

When the motor 18 is operating at f₃, the same conditions are evaluated,as noted above.

By implementing the fast stopping method in accordance with theinvention, the motor load combination will come to a quick stop. Thecurrent into the motor 18 shows large increase during this rapidstopping method. The level of current flowing into the motor directlydepends on the modified V/F pattern. Larger values of V₂ at f₂, in FIG.6, results in larger current, limited only by the hardware current limitof the inverter 28. Larger current can cause higher temperature rise inthe motor 18, which can affect performance. The stopping time achievablewith a larger value of V₂ is much faster than with a smaller value ofV₂. The selection of the value for V₂ at f₂, in FIG. 6, is thus veryimportant and is application-dependent.

It has been observed that, if during the entire stopping method thedifference between the actual speed and the commanded speed is high,indicating high slip, the motor successfully achieves low speed withoutcreating DC bus over voltage. Advantageously, high slip is maintainedbetween the actual speed and the commanded speed. The factors thatinfluence the desired high slip condition are over voltage sense level,undercurrent sense level, modified V/F pattern and operating temperatureof the motor. The DC bus voltage level should be such that it is muchlower than the regular over voltage trip level so that the second jumpis initiated much before the actual over voltage trip for the inverter.If this is not done, then the inverter faults out and all control islost. The undercurrent sense level should be higher than the no-loadmotor current or else this will never be initiated and the logic becomesineffective. If the voltage at the first and second jump frequencies ishigh, then the stator current is high. This can lead to over currenttrip of the inverter. On the other hand, a lower voltage value resultsin longer stopping time with lower stator current. The stopping time canbe optimized by judiciously choosing the voltage level at the first andsecond jump frequencies.

The magnitude of stator current flowing into the motor and consequentlythe stopping time depend on the rotor and stator resistances. Both ofthese values in turn depend upon the operating temperature. Higheroperating temperatures cause the point of zero power return to shifttowards larger slip values. Hence, the first and second jump frequenciesshould be selected to be much lower than the point of zero power return.

Experimental test results using the fast stopping method in accordancewith the invention show substantial improvement. A 3.7 kilowatt motorwith no brake resistor unit in the inverter, operating at 80 Hz with aload-inertia of about 8 times its rotor inertia, has a typical stoppingtime of about 9.57 seconds. The best stopping achievable using the faststopping method is on the order of 2.79 seconds. The fast stoppingmethod with a modified V/F pattern with current held to 120 percent ofinverter rating has a stopping time on the order of 7.278 seconds. Bymanipulating the motor flux, the stator current is reduced. This isachieved by sacrificing the stopping time.

Thus, in accordance with the invention, a new method of braking inertialloads is implemented. The method involves maintaining large slip duringthe stopping of the motor. Large value of negative slip results in lowregenerative torque being produced. Most of the available mechanicalenergy is converted into heat in the rotor bars, stator windings, andstator core where it is dissipated. There is no use of external resistorbanks or encoders. The fast stopping method does not cause DC bus overvoltage. In order to control the rotor current in the fast stoppingmethod, the V/F control pattern is modified. The modification, which iseffective only for the fast stopping method does not interfere withnormal operation. The modification helps to tailor the speed dropprofile without causing over voltage trip and maintaining currentcontrol over the reflected rotor current. The effect of temperature onthe motor performance in the negative slip region, suggests that inorder to ensure trip-less operation, it is safer to operate the motor ata negative slip that is more negative than the point of zero powerreturn. The fast stopping method achieves phenomenally fast stoppingtime with good control over current without creating high DC bus voltageconditions in the inverter.

We claim:
 1. A control method for an induction motor operated by avariable frequency drive (VFD), comprising: controlling the VFD tooperate the induction motor at a select commanded operating speed bycontrolling output frequency of the VFD; and if stopping of theinduction motor is initiated, then controlling the VFD to operate theinduction motor at a high negative slip condition to rapidly stoprotation of the induction motor.
 2. The control method of claim 1wherein the step of controlling the VFD to operate the induction motorat a high negative slip condition comprises reducing commanded speed toabout fifty-five percent of rated speed.
 3. The control method of claim1 wherein the step of controlling the VFD to operate the induction motorat a high negative slip condition comprises reducing commanded speed toprovide negative slip of about −0.8.
 4. The control method of claim 1wherein the step of controlling the VFD to operate the induction motorat a high negative slip condition comprises reducing commanded speedcorresponding to a reduction of operating frequency to one-third ofrated output frequency.
 5. The control method of claim 4 wherein thestep of controlling the VFD to operate the induction motor at a highnegative slip condition comprises additionally reducing commanded speedcorresponding to a further reduction of operating frequency to one-thirdof reduced output frequency in response to a sensed stopping condition.6. The control method of claim 5 wherein the sensed stopping conditioncomprises one of DC bus over voltage, induction motor under current anda select lapsed time.
 7. A control for an induction motor operated by avariable frequency drive (VFD), comprising: a control unit adapted tocontrol the VFD in a normal mode to operate at a select commandedoperating frequency to control speed of the induction motor, and in astopping mode at a substantially lower commanded operating frequency tooperate the induction motor at a high negative slip condition to rapidlystop rotation of the induction motor.
 8. The control of claim 7 whereinthe control unit in the stopping mode controls the VFD to operate theinduction motor at a high negative slip condition by reducing commandedspeed to about fifty-five percent of rated speed.
 9. The control ofclaim 7 wherein the control unit in the stopping mode controls the VFDto operate the induction motor at a high negative slip condition byreducing commanded operating speed to provide negative slip of about−0.8.
 10. The control of claim 7 wherein the control unit in thestopping mode controls the VFD to operate the induction motor at a highnegative slip condition by reducing commanded operating frequency toone-third of rated output frequency.
 11. The control of claim 10 whereinthe control unit in the stopping mode controls the VFD to operate theinduction motor at a high negative slip condition by additionallyreducing commanded operating frequency to one-third of reduced outputfrequency in response to a sensed stopping condition.
 12. The control ofclaim 11 wherein the sensed stopping condition comprises one of DC busover voltage, induction motor under current and a select lapsed time.